A so-called "particle board" can be formed by compressing a layer of particulate material, generally a mixture of cellulosic or other particles with a binder, in a platen or other press at a temperature sufficient to activate the binder.
Customarily the particles consist predominantly or at least partially, if not entirely, of wood chips, splinters, sawdust and fibers, while the binder may be a thermally activatable synthetic resin which is admixed with the particles or is intrinsically present in the wood. For example, it is known that wood contains abietic resins which can assist in bonding particles of wood together under heat and pressure. In the case of synthetic-resin binders, use is preferably made of phenol-formaldehyde, urea, resorcinol or melamine resin.
When a layer or mat of the particulate material is subjected to heating and compression, the binder is activated (caused to flow and/or set) to bind the particulate material into a coherent sheet or board.
Depending upon the preparations of binder and particulate matter, the nature of the particles and binder, the pressure which is applied and the heating cycle, the boards can be relatively dense, rigid and impermeable to fluid for use as structural members of high strength, can be of moderate density, strength and porosity for use as wall-facing members, or highly porous and very low density for use as insulating boards.
In order to produce such boards, a layer of the particulate material or matter thereof must be applied to a mat-forming surface, which can be the upper surface of a conveyor or a mat-transfer or pressing sheet or plate.
The mat can be produced by metering the quantity of the particulate matter from a hopper and depositing it upon a moving surface which is displaced beneath a distributing head which insures a homogeneous deposition of the particulate material on the surface over the width desired. Such distributing heads are recommended because they prevent localized piling of the particulate material on the surface and serve to distribute the particles substantially uniformly over the width thereof.
It has been proposed (see German published application -- OFFENLEGUNGSSCHRIFT -- DT-OS 2 229 147) to provide the distributing head with a plurality of so-called disk rollers with the disks of the successive rollers interdigitating. The resulting array has its disks or rollers driven in the same sense so that the particulate material is carried along the top of the array and eventually passes between the disks or rollers through spaces between them or other mat-forming surfaces.
The conventional apparatus has not been used, to our knowledge, for the formation of particle-board mats although the literature does describe a system in which hydraulic binders and fibrous material are deposited in a layer for the production of plastic board. In this case, the disk rollers form part of a wetting device in which the mixture of hydraulic binder and fiber is wetted with water serving as the activator for the hydraulic binder. This system is intended exclusively for the uniform deposition of the mixture upon the receiving surface and there has not been, to our knowledge, any suggestion of the use of this device or any similar device in the fabrication of particle board.
In the production of particle board, moreover, it has been found to be advantageous to provide a surface region with a different porosity or particle characteristic than the body of the board. Such an arrangement has aesthetic reasons as well as structural reasons, the latter deriving from, for example, the lower porosity of a fine-particle zone and the surface of the board. The importance of a facing layer can also be seen from the fact that it has been proposed repeatedly to apply a coherent facing layer, foil or sheet to a particle board by conventional lamination techniques.
Heretofore, when a mat is to be formed with a surface zone of, say, fine particles, it has been necessary to use special means for applying a fine-particle-layer face to the receiving surface and thereafter depositing the rest of the particulate material thereon. Such techniques have proved to be expensive, time-consuming and cumbersome and do not always result in a highly homogeneous or uniformly faced structure.